1. Field of the Invention
This invention relates to methods of forming radially expandable medical articles through deformation of a polymeric material.
2. Description of the State of the Art
This invention relates to medical article such as expandable medical devices used in the treatment of diseased portions of bodily lumen. Expandable medical devices may include certain kinds of implantable endoprosthesis and inflatable members. Inflatable members such as balloons are used, for example, in angioplasty procedures or in implantation of endoprostheses which are adapted to be implanted in a bodily lumen. An “endoprosthesis” corresponds to an artificial device that is placed inside the body. A “lumen” refers to a cavity of a tubular organ such as a blood vessel. A stent is an example of such an endoprosthesis. Stents are generally cylindrically shaped devices which function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of the diameter of a bodily passage or orifice. In such treatments, stents reinforce body vessels and prevent restenosis following angioplasty in the vascular system. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been treated (as by balloon angioplasty or valvuloplasty) with apparent success.
The treatment of a diseased site or lesion with a stent involves both delivery and deployment of the stent. “Delivery” refers to introducing and transporting the stent through a bodily lumen to a region, such as a lesion, in a vessel that requires treatment. “Deployment” corresponds to the expanding of the stent within the lumen at the treatment region. Delivery and deployment of a stent are accomplished by positioning the stent about one end of a catheter, inserting the end of the catheter through the skin into a bodily lumen, advancing the catheter in the bodily lumen to a desired treatment location, expanding the stent at the treatment location, and removing the catheter from the lumen. In the case of a balloon expandable stent, the stent is mounted about a balloon disposed on the catheter. Mounting the stent typically involves compressing or crimping the stent onto the balloon. The stent is then expanded by inflating the balloon. The balloon may then be deflated and the catheter withdrawn. In the case of a self-expanding stent, the stent may be secured to the catheter via a retractable sheath or a sock. When the stent is in a desired bodily location, the sheath may be withdrawn which allows the stent to self-expand.
The stent must be able to satisfy a number of mechanical requirements. First, the stent must be capable of withstanding the structural loads, namely radial compressive forces, imposed on the stent as it supports the walls of a vessel lumen. Therefore, a stent must possess adequate radial strength. Radial strength, which is the ability of a stent to resist radial compressive forces, is due to strength and rigidity along the circumferential direction of the stent. Radial strength and rigidity, therefore, may be also be described as, hoop or circumferential strength and rigidity. Additionally, the stent should also be longitudinally flexible to allow it to be maneuvered through a tortuous vascular path and to enable it to conform to a deployment site that may not be linear or may be subject to flexure. The material from which the stent is constructed must allow the stent to undergo expansion which typically requires substantial deformation of localized portions of the stent's structure. Once expanded, the stent must maintain its size and shape throughout its service life despite the various forces that may come to bear on it, including the cyclic loading induced by the beating heart. Finally, the stent must be biocompatible so as not to trigger any adverse vascular responses.
The structure of a stent is typically composed of scaffolding that includes a pattern or network of interconnecting structural elements or struts. The scaffolding can be formed from wires, tubes, or sheets of material rolled into a cylindrical shape. The scaffolding is designed to allow the stent to be radially expandable. The pattern should be designed to maintain the longitudinal flexibility and radial rigidity required of the stent. Longitudinal flexibility facilitates delivery of the stent and radial rigidity is needed to hold open a bodily lumen.
Stents have been made of many materials such as metals and polymers, including biodegradable polymer materials. A medicated stent may be fabricated by coating the surface of either a metallic or polymeric scaffolding with a polymeric carrier that includes an active agent or drug. In many treatment applications, the presence of a stent in a body may be necessary for a limited period of time until its intended function of, for example, maintaining vascular patency and/or drug delivery is accomplished. Therefore, stents fabricated from biodegradable, bioabsorbable, and/or bioerodable materials such as bioabsorbable polymers may be configured to meet this additional clinical requirement since they may be designed to completely erode after the clinical need for them has ended.
Conventional methods of constructing a stent from a polymer material involve extrusion of a polymer tube based on a single polymer or polymer blend and then laser cutting a pattern into the tube. An advantage of stents fabricated from polymers is that they can possess greater flexibility than metal stents. Other potential shortcomings of metal stents include adverse reactions from the body, nonbioerodability, and non-optimal drug-delivery. However, a disadvantage of polymer stents compared to metal stents, is that polymer stents typically have less circumferential strength and radial rigidity. Inadequate circumferential strength potentially contributes to relatively high recoil of polymer stents after implantation into vessels. Another potential problem with polymer stents is that struts can crack during crimping, especially for brittle polymers. Furthermore, in order to have adequate mechanical strength, polymeric stents may require significantly thicker struts than a metallic stent, which results in an undesirable larger profile. Therefore, methods of manufacturing polymer stents that improve circumferential strength and radial rigidity are desirable. The embodiments presented herein address the issue of improving circumferential strength and radial rigidity in polymer stents.
As indicated above, inflatable members may include angioplasty and stent delivery balloons. Angioplasty and stent delivery balloons are typically made of polymeric materials. In general, the polymeric material is extruded into tubular shapes or parisons. The extruded parison is then formed into the balloon shape using a blow molding process. A balloon blow molding process includes a mold, a temperature source, a pressure source, and a tension source. In the balloon molding process, the extruded tubing is placed inside the mold and subsequently the mold is heated with the temperature source. The tubing may be stretched longitudinally under the influence of the tension source and is expanded under the influence of the pressure source. The pressure source typically consists of a nozzle connected to one end of the parison. The nozzle is configured to blow air into the parison to expand the parison within the confines of the mold. The final balloon shape is primarily determined by the geometric design of the mold and process parameters.
Furthermore, high circumferential strength and modulus are also extremely important for inflatable members, such as catheter balloons for use in angioplasty procedures and for delivering stents. Additionally, thinner walls are also strongly desirable for inflatable members since a low form factor of the balloon facilitates transport of the balloon through a vessel. Methods described above that are typically used for forming inflatable members do not allow adequate control over circumferential strength and modulus, as well as wall thickness. In addition, such methods are unable to fabricate balloons of a desired size out of some materials. Failure of the expanding parison often occurs during fabrication. The shortcomings of inflatable member fabrication are addressed by embodiments presented herein.